1. Field of the Invention
The present invention relates to an arrangement of a thin film semiconductor device using a crystal silicon film and to a fabrication method thereof.
2. Description of Related Art
Hitherto, there has been known a thin film transistor using a silicon film, i.e. a technology for forming the thin film transistor by using the silicon film formed on a glass substrate or quartz substrate.
The glass substrate or quartz substrate is used because the thin film transistor is used for an active matrix type liquid crystal display. While a thin film transistor has been formed by using an amorphous silicon film in the past, it is being tried to fabricate the thin film transistor by utilizing a silicon film having a crystallinity (referred to as “crystal silicon film” hereinbelow) in order to enhance its performance.
The thin film transistor using the crystal silicon film allows to operate at higher speed by more than two digits as compared to one using the amorphous silicon film. Therefore, peripheral driving circuits of an active matrix type liquid crystal display which have been composed of external IC circuits may be built on the glass substrate or quartz substrate in the same manner with the active matrix circuit.
Such structure is very advantageous in miniaturizing the whole apparatus and in simplifying the fabrication process, thus leading to the reduction of the fabrication cost.
In general, a crystal silicon film has been obtained by forming an amorphous silicon film by means of plasma CVD or reduced pressure thermal CVD and then by crystallizing it by implementing a heat treatment or by irradiating laser light.
However, it has been the fact that it is difficult to obtain a required crystallinity throughout a wide area through the heat treatment because it may cause nonuniformity in the crystallization.
Further, although it is possible to obtain the high crystallinity partly by irradiating laser light, it is difficult to obtain a good annealing effect throughout the wide area. In particular, the irradiation of the laser light is apt to become unstable under the condition for obtaining the good crystallinity.
Meanwhile, a technology described in Japanese Patent Laid-Open No. Hei. 6-232059 has been known. This is a technology which allows to obtain a crystal silicon film through a heat treatment at a lower temperature than that of the prior art by introducing a metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film.
This technology allows high crystallinity to be obtained uniformly throughout a wide area as compared to the prior art crystallization method by way of only heating or crystallization of an amorphous silicon film only by means of irradiation of laser light.
However, it is difficult to obtain a crystal silicon film having high crystallinity and homogeneity throughout a wide area which is required for an active matrix type liquid crystal display.
Further, because the metal element is contained within the film and an amount thereof to be introduced has to be controlled very carefully, there is a problem in its reproducibility and stability (electrical stability of a device obtained).
Still more, there is a problem that an elapsed change of the characteristics of a semiconductor device to be obtained is large or an OFF value, in case of a thin film transistor, is large, for example due to the influence of the remaining metal element.
That is, although the metal element which promotes the crystallization of silicon plays the useful role in obtaining the crystal silicon film, its existence becomes a minus factor which causes various problems after obtaining the crystal silicon film once.